1. Field of the Invention
The present invention relates to an image transmission system, and more particularly to such system in which plural image processing devices such as an image reading device or an image recording device are mutually connected through a signal transmission channel such as optical fibers for transmitting image information among such devices.
2. Description of the Prior Art
There are already proposed or in use various information transmission systems in which plural information processing devices are mutually connected through signal transmission channels for mutual exchange of information among such devices or information distribution from one device to others.
Also recently proposed is the information transmission in the form of optical signals through optical fibers instead of the conventional information transmission by electrical signals. Such transmission through the optical fibers avoids electromagnetic perturbation from the exterior, allows high-speed transmission and facilitates the extention of transmission channels because of the lighter weight of such optical fibers.
Information can be largely classified into two categories, one being data information only identifiable by computers, and the other being image information only identifiable through human perception. The conventionally proposed transmission through optical fibers relates only to the data information and is achieved through time-sharing means among plural devices mutually connected by optical fibers. However such time-sharing means has a limitation in the amount of information transmittable per unit time, and is therefore unable to fully exploit the advantage of the optical fibers as the medium for high-speed transmission.
Also clear transmission of an image with a high resolving power requires a large amount of image information per original document, which is preferably transmitted in continuous manner in consideration of the nature of image information. For this reason the optical fibers have to be occupied for a long time if such image information is transmitted through the time-sharing means.
Furthermore such time-sharing means often requires the use of a complicated separating circuit for identifying plural information present on the optical fibers.
Besides, in case of transmitting image information from an image processing device to plural similar devices, they are seldom in a position to receive the information simultaneously because of the difference in the status thereof. For this reason the transmission is usually achieved by once storing the image information to be transmitted in a memory in the transmitting device, and by thereafter transmitting the information from said memory to each device in a status capable of receiving the information. However such transmission method with a resolving power as high as 16 pel/mm or 16 line/mn requires a memory capacity of 15,996,720 bits for an image of A4 size, and such memory is too costly in order to be equipped in all the devices transmitting the information.
Although there are known various transmission systems for signal transmission through optical fibers, a loop transmission system is advantageous in consideration of the length of optical fiber required for connecting plural terminal devices and of the transmission speed. Such loop transmission system is disabled if the optical fiber constituting the loop is broken for some reason. In order to rapidly cope with such failure each device belonging to the loop is provided with means for detecting the breakage of the loop by detecting the return of a signal emitted by said device, but a particular protocol is required for such loop failure detection.
Also in such a loop transmission system, the collision of plural signal packets from the different terminal devices is avoided by providing the devices in the loop with semi-fixed priorities, according to which the demands for transmission are identified and ordered. Such system however requires a particular circuit for identifying the priorities, and it is not easy to change such priorities.